Update SparkFun_ADXL345_Example.ino

[FroggyCorp]

There are multiple problem on using interrupt :

• The ADXL_ISR() is too long and crash the arduino, i put only a flag
• After reading internet, we need an adxl.getInterruptSource(); after define the interrupt for arduino (or before ?). If not, int will never trigger. We also need an adxl.getInterruptSource() to refresh the int of ADXL.
• I add a pinmod for safety (arduino is 5V and ADXL345 is 3V)